Question decision of union boss trusting leaders (5)
I believe the answer is:
doubt
'question' is the definition.
(I've seen this before)
'decision of union boss trusting leaders' is the wordplay.
'leaders' suggests taking the first letters.
The initial letters of 'decision of union boss trusting' is 'DOUBT'.
(Other definitions for doubt that I've seen before include "Suspect" , "Question the truth or accuracy of" , "Disbelieve (someone)" , "Distrust" , "Uncertainty, unsureness" .)